FIG. 1 illustrates a prior art system for converting DC power from a photovoltaic (PV) panel to AC power. The PV panel 10 generates a DC output current IPV at a typical voltage VPV of about 35 volts. A DC/DC converter 12 boosts VPV to a link voltage VDC of about 420 to 450 volts DC. A DC/AC inverter 14 converts the DC link voltage to an AC output voltage VGRID. In this example, the output is assumed to be 240 VAC at 60 Hz to facilitate connection to a local power grid, but other voltages and frequencies may be used.
The system of FIG. 1 also includes a DC link capacitor CDC and a decoupling capacitor C1. Either or both of these capacitors may perform an energy storage function to balance the nominally steady power flow from the PV panel with the fluctuating power requirements of the grid. Power ripple within the system originates at the DC/AC inverter 14, which must necessarily transfer power to the grid in the form of 120 Hz ripple. In the absence of a substantial energy storage device, this current ripple would be transferred all the way back to the PV panel where they would show up as fluctuations (or “ripple”) in the panel voltage VPV and/or current IPV. Therefore, the DC link capacitor CDC or the decoupling capacitor C1, is used to store enough energy on a cycle-by-cycle basis to reduce the ripple at the PV panel to an acceptable level. In a typical system, the DC link capacitor may operate with a ripple component of about 100 volts peak-to-peak and may use sophisticated algorithms to control the power flow with a sinusoidal grid voltage and corresponding sinusoidal grid current.
The system of FIG. 2 typically includes maximum power point tracking (MPPT) functionality that forces the DC/DC converter to operate at an input voltage and current that maximizes the total amount of power available from the PV panel under its specific operating conditions.
FIG. 1 illustrates the internal structure of a prior art PV module. A typical module includes three substrings of PV cells, internally connected in series and bypassed with diodes. Each substring typically generates a voltage in the range of 10 to 20 volts depending on the amount of sunlight it receives. If a substring becomes shaded, its bypass diode conducts, totally eliminating any power harvest from that substring and reducing the output voltage of the PV module by one-third. FIG. 3 illustrates equivalent circuit of a substring which may be modeled as a current source shunted by numerous diodes connected in series.
A problem with the prior art is that, with the substrings connected in series, the same amount of current must flow through each substring. However, since the substrings may be subjected to different amounts of sunlight due to shading or other conditions, the maximum power point for each substring may require each substring to operate at a different current level. Thus, one or more substrings cannot operate at its maximum power point.
A further problem is that, with three substrings connected in series, each generating power at 10-20 volts, the DC/DC converter must be able to accommodate maximum input voltage of about 70 volts. However, if two substrings are shaded, the minimum operating voltage for the panel may be as low as 10 volts. Thus, the DC/DC converter must be able to accommodate a 10-60 volt input voltage range. Among other problems, this places difficult demands on the DC/DC converter, reduces efficiency, and requires more expensive solid state switches to tolerate the high input voltages.